1. Technical Field
The invention relates generally to precision measurement instruments, and more particularly to systems and methods for operating a chromatic point sensor (CPS) pen to measure a characteristic of a hole, such as a hole diameter or the like, without having to rotate the CPS pen.
2. Description of the Related Art
Axial chromatic aberration techniques may be utilized for distance sensing metrology. As described in “Pseudocolor Effects of Longitudinal Chromatic Aberration”, G. Molesini and S. Quercioli, J. Optics (Paris), 1986, Volume 17, No. 6, pages 279-282, controlled longitudinal chromatic aberration (also referred to herein as axial chromatic dispersion) may be introduced in an optical imaging system, causing the imaging system focal length to vary with wavelength, which provides means for optical metrology. In particular, a lens can be designed whose back focal length (BFL) is a monotonic function of wavelength. In white light operation, such a lens exhibits a rainbow of axially dispersed foci that can be used as a spectral probe for distance sensing applications.
As a further example, U.S. Pat. No. 7,477,401, which is incorporated herein by reference in its entirety, discloses that an optical element having axial chromatic aberration may be used to focus a broadband light source such that the axial distance or height of a surface determines which wavelength is best focused at that surface. Upon reflection from the surface, the light is refocused onto a small detector aperture, such as a pinhole and/or the end of an optical fiber, and only the wavelength that is well-focused on the surface is well-focused on the aperture. Other wavelengths are poorly focused and will not couple much power into the aperture. A spectrometer measures the signal level for each wavelength returned through the aperture. A wavelength intensity peak effectively indicates the distance or height of the surface.
Certain manufacturers refer to a practical and compact optical assembly that is suitable for chromatic confocal ranging in an industrial setting as a chromatic confocal point sensor, a chromatic point sensor (CPS) including an optical pen and/or as simply an “optical pen.” One example of optical pen instruments that measure Z height are those manufactured by STIL, S.A. of Aix-en-Provence, France (STIL S.A.). As a specific example, the STIL optical pen model number OP 300NL measures Z heights and has a 300 micron range.
Another configuration for a chromatic confocal point sensor is described in commonly assigned U.S. Pat. No. 7,626,705, which is hereby incorporated herein by reference in its entirety. This patent discloses a lens configuration providing an improved optical throughput and an improved spot size, which results in improved measurement resolution in comparison with various commercially available configurations.
Applications of currently available optical pens to measure characteristics of a hole, such as a hole diameter, require rotating the optical pen relative to the hole to be measured in order to measure different portions of the hole wall and determine its overall shape and/or location. Pen rotation requires complex opto-mechanical parts to effect the rotation. In various applications, rotation of the pen is time-consuming and lowers measurement throughput. Also, rotation of the pen inevitably introduces some level of runout and/or wobble and related measurement errors.
Commonly assigned U.S. Pat. No. 8,194,251 (“the '251 patent”), which is incorporated herein by reference in its entirety, discloses a dual beam optical pen which may be positioned to measure two surface regions simultaneously. Generally speaking, while the dual beam configurations disclosed in the '251 patent may provide certain advantages relative to single beam optical pens, the pens in the '251 patent are not specifically configured for reliable and efficient hole measurement, particularly for measuring hole diameters that may approach the pen diameter.
For various applications involving measurement of holes and/or their interior surfaces, improvements in the configuration of and method of using CPS optical pens may be desirable. The present invention is directed to providing systems and methods for operating a CPS optical pen to measure a characteristic of the interior surfaces of a hole, such as a hole diameter, without having to rotate the pen.